Diamond electrodes of this type are produced in a known manner by a diamond layer being built up on a metallic electrode body under defined ambient conditions. The problem with this process is that a stable connection has to be ensured between the diamond layer and the electrode body. In order to improve this connection, it is known to bombard the surface of the electrode body with particles of a size of from 20 to 100 μm by means of a sand blasting process in order to thereby produce a topographically nonuniform surface—and therefore an enlarged contact surface with the diamond layer. The particles used in this process may be formed from any sufficiently hard material that does not react with the material of the electrode body. A preferred material is sic.
In some applications, nevertheless, the adhesion of the built-up diamond layer to the electrode body having a surface roughened in this way does not have the desired stability. In particular, spalling of the diamond layer from the electrode body may occur.
Investigations have revealed that one cause of the spalling is that a not inconsiderable quantity of sand blasting particles are mechanically anchored in the surface of the electrode body during the sand blasting process, and therefore there are between 103 and 104 blasting particles in each square centimeter of the electrode surface after the sand blasting. In this process, the blasting particles are more or less firmly anchored in the surface, in which case the loosely anchored blasting particles could represent a cause of the spalling of the diamond layer which has been built up.
It has therefore been proposed to etch the surface using a conventional oxidative etchant, such as hydrofluoric acid (HF) or nitric acid (HNO3), after the sand blasting. In fact, etching with hydrofluoric acid has succeeded in reducing the number of blasting particles per square centimeter of electrode surface to below 5. Nevertheless, this measure has not resulted in improved diamond electrodes. Firstly, a metal oxide is formed during etching, i.e. niobium oxide in the case of a preferably used electrode body composed of niobium, and this permits only impaired adhesion of the diamond layer. Furthermore, the electrode surface which is embrittled during etching results in delamination of the diamond layer.
A problem present in the prior art is therefore that many of the diamond electrodes produced become unusable owing to spalling of the diamond layer, and therefore thorough testing has to be carried out. This makes the diamond electrodes considerably more expensive. In addition, the diamond electrodes have to be handled very carefully since they also tend toward spalling of the diamond layer when in use.